1. Field of the Invention
The present invention relates to a process cartridge that includes a development device and is incorporated in an image forming apparatus such as a copier, a printer, a facsimile machine, a plotter, a multi-function machine, and the like.
2. Discussion of the Background
In general, development devices that develop toner images for electrophotographic printing employ either one-component developer or two-component developer. Structurally, known development devices using two-component developer consisting essentially of toner and magnetic carrier generally include a development casing and multiple toner screw conveyers supported by bearings formed in the casing to agitate and transport supplied toner and carrier, with the toner screw conveyers generally formed of a shaft with a bladed spiral portion so as to transport the toner unidirectionally. A first toner screw conveyer and a second toner screw conveyer are respectively provided in a development roller chamber and an agitation chamber. As the toner is consumed, more toner is supplied from an upper portion of the agitation chamber in a direction in which the toner is transported (hereinafter “toner transport direction”). The replenished toner is mixed with the carrier in the agitation chamber and is conveyed toward a downstream portion of the agitation chamber along the spiral portion of the toner screw conveyor.
In the development devices including the above-described toner screw conveyers, unless the shaft of each toner screw conveyer is almost perfectly round the toner may leak from between the bearings and the toner screw conveyor. Therefore, a collar or the like that can be perfectly round relatively easily is fitted around the toner screw conveyor between the bearing and the toner screw conveyor.
Additionally, in order to fit the collar around the screw shaft of the toner screw conveyer without damaging the screw shaft by pressing tools, a flange portion may be provided on the collar.
In the known development devices configured as described above, a driving mechanism to rotate the screw conveyors and a development roller is provided on the outside of the development casing. The driving mechanism generates heat due to the pressure angle of gears engaging each other and the friction of sliding portions, such as the bearings. With the above-described collar positioned between the bearing and the toner screw conveyor, the heat generated by driving is transmitted to the interior of the development casing, increasing the temperature thereof.
Further, the toner is conveyed by the spiral portion of the second toner screw conveyer downstream in the toner transport direction, and then is sent to the development roller chamber by a rib that is located on the screw shaft of the second toner screw conveyer and extending parallel to the screw shaft. Consequently, it can happen that toner can accumulate in a given portion in the development device and is not transported by the toner screw conveyer due to a difference between sending pressure and returning pressure and changes in conveyance speed. Such accumulated toner generates pressure (toner powder pressure) on the extreme downstream portion of the screw shaft, the toner powder pressure presses the flange portion of the collar extending around the screw shaft in a direction orthogonal to the screw shaft, and the collar easily becomes hot. When the temperature of the collar reaches the melting point of the toner, the toner may coagulate, and then is fused and adhered to the flange portion, which is a phenomenon called fusing adhesion.
The coagulated toner formed on the flange portion of the collar can drop therefrom when the driving mechanism stops or the like, and the toner fragment thus dropped can easily separate into smaller pieces. If these fragments get to the development roller, the development roller may not carry the toner uniformly, which is a problem in that it adversely affects image quality.
In view of the foregoing, there is market demand for a development device capable of preventing fusing adhesion caused by the pressure and the temperature in the development device.